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From: "torvalds@linux-foundation.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/113921] Output register of an "asm volatile goto" is
 incorrectly clobbered/discarded
Date: Wed, 14 Feb 2024 18:25:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113921

--- Comment #5 from Linus Torvalds <torvalds@linux-foundation.org> ---
(In reply to Linus Torvalds from comment #2)
>
> So we could make our workaround option be something like
>=20
>    config GCC_ASM_GOTO_WORKAROUND
>      def_bool y
>      depends on CC_IS_GCC && GCC_VERSION < 120100

Replying to myself some more, since that kernel side workaround is actually=
 in
two parts: it does the extra empty inline asm as an extra barrier, but it
*also* adds the 'volatile' to the asm with outputs to work around the other=
 gcc
bug.

And that other fix ("Mark asm goto with outputs as volatile") is *not* in
gcc-12.1.0. It has only made it into gcc-13.2.0 (and it's pending in the gc=
c-12
release branch if I read things right).

I suspect that other bug doesn't affect Sean's kvm case, because the outputs
are always used, but it could affect other kernel code.

So we'd want to have at least gcc-13.2 to be safe.

Hmm.

We could make the "add volatile manually" be the default workaround, though,
since it shouldn't matter.=